1. Field of the Invention
This invention relates to a device and process for determining the state of a runway, as well as to an aircraft's takeoff and/or landing assistance system and process, and to aircraft equipped with such devices and systems.
2. Discussion of Prior Art
During landing and takeoff phases, and more generally when a plane is on the take off or landing roll, being aware of the state of the runway surface is of the utmost importance. Indeed, predicting the plane's braking performance depends on said awareness. Thus it is possible: —To best estimate the distance needed to stop the plane in a concern for safety, —To not overestimate this stopping distance needed to bring the plane to a standstill and therefore to not overly penalize the runway and plane's utilization operations.
Yet, a plane's braking performances on a runway said to be contaminated are very difficult to predict because of the difficulty in having a reliable and accurate knowledge of the runway's contribution to the plane's deceleration, in particular in terms of adhesion and projection drags and displacements in the case of thick contaminants. Contaminants can be any item that comes to rest on the “original” runway, such as for example, rubber left during previous landings, oil, rainwater forming a more or less uniform layer on the runway, snow, ice, etc. Knowledge of such a contribution to the state of the runway may seem beneficial to improve landing systems such as the one described for example in the document FR-2897593. This knowledge may also prove to be important to make the take-off of planes more secure, as the latter must estimate, for example, the point of no return of the runway, where a completely safe emergency braking on the remaining part of the runway is no longer possible. Initial solutions to estimate the state of the runway have already been implemented, but today, the runway's adhesion measurements are very difficult, not very effective, not reliable and hard to transpose from the context of the measurement means used to that of a plane on the take off or landing roll on the same runway.
In particular, complete and reliable solutions should estimate a runway state: —independently of the measurement means used; —useable by any other plane, and in particular by its landing system; —updated on a regular basis; —correlated with a runway position; —for several runway points or portions in order to cover the entire runway; —without needing to close the runway.
In particular, it is known that the measurement of the runway's adhesion thanks to friction engines or “mu-meters”, for example towed vehicles or special vehicles, that provide results that are ill-assorted, potentially inconsistent among themselves, not representative for an airplane because of the different scales of phenomena such as the loads and tire performance. Therefore, these results are not used by the other planes, but only internally by the airports in order to obtain an idea of the deterioration of the runway. In addition, such measurements require closing the runway for several minutes, which may be harmful considering the ever increasing airport traffic. The estimate of a runway's state by “mu-meters” is therefore not usually updated on a regular basis.
In practice one also resorts to visual and manual inspections of the runway at several points thereof, by an agent, in order to obtain a type and thickness of the contaminant along the runway. However, this approach only provides an indication that is strongly based on the location where the inspection was carried out. Furthermore, it requires closing the runway for long periods of time (around twenty minutes), which is not compatible with airport traffic. The estimate of the runway's state is therefore not updated on a regular basis using a visual inspection.
Also, “Reported Braking Actions” or “Pilot Reports” (PIREP) are actually what the plane's pilot experiences about his braking performances with a division into several simple levels of evaluation: for example good/good-medium/medium/medium-poor/poor/nil (in practice indicated according to the following English terms: “good”/“good-medium”/“medium”/“medium-poor”/“poor”/“nil”) from which it is possible to manually notify the plane on approach for landing. In airports with traffic, as the planes follow each other for example every two minutes, regular estimations are therefore obtained.
However, this solution is subjective, depends on the plane and takes into account contributions other than wheel braking (as the pilot is not able to identify the exact part of the various means of braking of his plane: aerodynamic drag, engine thrust or counter-thrust and wheel braking).
On the contrary, this invention concerns a solution for estimating the state of the runway that is more reliable, in particular objective and representative of airplane behavior. In this area, analysis solutions applicable later on the ground have already been developed to estimate a posteriori the state of the runway at the time of an incident or an accident during service, or to validate trial flights in “real time”. These solutions are usually based on measurements of the plane's deceleration during landing. Then, on the ground, delayed treatments are performed to estimate the adhesion of the runway based on this measured deceleration, by subtracting in particular aerodynamic, engine and contaminant components or contributions, resulting from models using other measurements performed on or outside the airplane.
These treatments performed, take into account the type of plane involved because the measurement of the deceleration alone does not allow for an easy utilization by another plane. In addition, these treatments are long, manual and not compatible with an intensive use of an airport where an estimation of the runway state is required in a brief period before the next plane performs a take off or landing roll on the runway.
Furthermore, document US 2006/243857 is a method and a device to estimate features pertaining to a landing runway. A real time treatment is carried out during which various airplane or external parameters are acquired and recorded. From these recorded parameters, an estimation of the deceleration due solely to braking is performed in particular based on the deceleration Ax of the airplane, the engine thrust Areverse thrust, and the aerodynamic drags Adrag. A friction profile “μ” is then established in order to determine if the plane is at braking limit or not, and warn the pilot accordingly.
However, this information is simply not useable in a satisfactory timeframe to notify the approaching planes.
In the publication FR 2 930 669 (also published under the reference US 2009/029483), such an adhesion or friction profile μ (usually identified as “friction curve” in the literature) obtained for the current airplane is compared to a set of pre-established adhesion profiles in order to obtain a characterization of the runway state.
One difficulty in determining the state of the runway resides in the evaluation of an adhesion coefficient μ value concerning the physical environment of the plane, where this physical environment not only concerns the runway but also the plane itself (for example its tires).
For example, a small plane may experience a maximum adhesion μ of 1.3 on a dry runway and of 0.8 on a wet runway, while a large carrier (Very Large Aircraft or VLA) experiences a maximum adhesion μ of 0.6 on the same dry runway and of 0.4 on the same wet runway.
In this example, the 0.6 value of μ does not necessarily indicate a dry runway. When this is the case, it is for a specific plane, at a certain speed, using specific tires and landing at a certain temperature.
From these examples, the sole estimation of an adhesion coefficient μ does not make it possible to correlate, in a reliable and precise manner, the measurements with a runway state.